In an electrical power plant utilizing steam operated turbines, a major portion of the steam condensate is recycled and reused to generate additional steam. The condensate can contain dissolved carbon dioxide and through the recycling, the carbon dioxide content can increase. It has been found that dissolved carbon dioxide can be corrosive to the turbines, as well as other metal components of the power plant. Also, dissolved carbon dioxide can contribute to scale accumulation which can decrease the thermal efficiency of heat exchange surfaces. Therefore, it is desirable to measure the carbon dioxide content in boiler condensate so that additional makeup water can be added to the condensate to maintain the carbon dioxide content within required limits.
It is recognized that the total carbonate species in an aqueous carbonate system can consist of carbon dioxide, bicarbonate, carbonate, and carbonic acid, and the equilibrium concentrations of the various carbonate species in aqueous solution are controlled by the pH of the solution. More specifically, at a pH below about 4.5, the carbonate species will consist entirely of carbonic acid. As the pH is increased to a value of about 8.5, the carbonic species will consist entirely of bicarbonate and as the pH is raised above 8.5, the predominant carbonate species will be carbonate.
Various devices have been employed in the past to measure the dissolved carbon dioxide content in an aqueous medium. One method of determining the carbon dioxide content is through bicarbonate titration, in which the pH of the water sample is initially adjusted to a value of 8.5, to convert the total carbonate to the bicarbonate form. The solution is then titrated to a pH of 4.5, converting the bicarbonate to carbonic acid, and the amount of titrating solution required to achieve this pH is a measure of the carbon dioxide content.
Differential titration is similar to bicarbonate titration, in that it relies on initial adjustment to a pH value of 8.5 to convert the total carbonate concentration to bicarbonate. The sample is then titrated to a pH of 5.0 to produce carbon dioxide, and the carbon dioxide is then removed from the sample by heating to the boiling point. The sample is then readjusted to the pH value of 8.5 and titrated to a pH of 5.0. The difference between the two titrations allows the total starting carbonate free of non-volatile acid/base interferences to be determined.
Another method that has been used to measure the free carbon dioxide present in an aqueous sample is direct titration. In contrast to the above two methods that measure the total carbonate concentration, this method only measures the free carbon dioxide present in the sample. The sample is titrated with sodium hydroxide to a pH of 8.5 to convert the total carbonate to sodium bicarbonate, and assumes the carbon dioxide is the only acidic species present in the sample.
Electrical conductivity can be used to measure the content of carbon dioxide in water, when carbonate species are the predominant anions, and the pH is confined to a narrow range. In this situation, the conductivity effectively monitors the carbonate concentration. However, the cell must be standardized with a reference solution of known conductivity to calculate a cell constant.
Another device that has been used commercially for monitoring carbon dioxide in condensate water at ppm levels is the Severinghaus type probe. This probe consists of a potentiometric glass pH probe and a thin layer of bicarbonate/sodium chloride filling solution is enclosed between the outer glass surface of the probe and a silicone rubber membrane. An internal reference electrode, which maintains its potential via the chloride content of the filling solution, is included. The analytical response in this probe is based upon the change in pH of the filling solution, due to carbon dioxide diffusion through the membrane into the thin layer of carbonate solution which produces a potentiometric signal.
A further device for measuring dissolved carbon dioxide based on potentiometry is the Scarano probe. In this probe, two electrodes of the same metal are utilized with a filling solution of pure water. A small relatively stable water layer is trapped between the working end of an indicator electrode and a gas permeable membrane formed of silicone rubber. Diffusion of carbon dioxide from the sample through the membrane to this water layer results in changes of surface pH at the electrode, and the change in metal surface oxidative potential resulting from the pH change determines the analytical signal.
While a number of devices for measuring the content of carbon dioxide in condensate have been utilized in the past, as described above, none of these devices have been capable of providing a rapid and stable response at sub-ppm levels, such as below 100 ppb.